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greatest prime factor

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Source: — Problem Solving |

Re: greatest prime factor

by umaa » Mon Oct 05, 2009 5:05 pm
DeeeJaaayyy wrote:What is the greatest prime factor of 4^17-2^28?

A) 2
B) 3
C) 5
D) 7
E) 11

Can i get some help on how to solve this?
IMO D.

4^17 = 2^34

2^34 - 2^28 = 2^28(2^6 - 1) = 2^28 (63).

The factors are, 1, 2, 7, 3.

The greatest prime number is 7.
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